在 reactor.netty WebSocketSession 中发送更多数据之前如何确保前面的 session.send() 完成

Question

I am creating a WebSocket Server using spring-webflux and reactor.netty, and on every connection, I get a WebSocketSession (with ReactorNettyWebSocketSession instance). Suppose I store all the sessions in a map and want to send some messages to this WebSocketSessions based on some business logic, I can do something like

            Flux<String> stringFlux = Flux.fromIterable(messages);
            for (WebSocketSession session : sessions.values()) {
                if (session.isOpen()) {
                    session
                            .send(stringFlux.map(session::textMessage))
                            .subscribe();
                } else {
                    System.out.println("session is closed.. skipping.. " + session.getId());
                    sessions.remove(session.getId());
                }
            }

Now, when I send messages to the session, is there a way to make sure that the session's previous send is complete? If a client is very slow and/or not reading from the server, it can create memory overhead for the server if the server keeps writing to the socket when the client is not reading or reading very slowly.

How to get a callback or some mechanism in which I can prevent writing to the WebSocketSession/socket if the client is slow?

Answer 1

There are quite a lot of assumptions behind this answer so we might need to discuss the details in the comments. The idea is that the following code sends messages one by one for each session, but concurrently for all sessions. You can tweak this according to your needs, turning concatMap to flatMap will enable concurrent sending while turning flatMap to concatMap will ensure that all all messages have to be sent to a single session before moving on to the next one.

Flux.fromIterable(sessions.values())
    .filter(session -> session.isOpen()) // keep open sessions
    .doOnDsicard(Session.class, session -> {
        System.out.println("session is closed.. skipping.. " + session.getId());
        sessions.remove(session.getId());
    })
    // flatMap here will allow the messages to be sent to each session concurrently
    .flatMap(session -> Flux.fromIterable(messages) 
        .map(session::textMessage)
        // concatMap here will force messages to be sent one by one (per session)
        .concatMap(message -> Mono
            .fromCallable(() -> session
                .send(Mono.just(message))), 0))